Lime mortar you can do very nicely with a mixing paddle attachment on
an sds drill (screwfix do the paddles). I measure out into a trug, mix
it there - keep it wrapped in a binbag - and knock it up again as I
need it.

I'm told lime mortar can be a bit too sticky for a mixer with the
blades in it - forms a clump that remains lodged - but that you can
cut the blades out with an angle grinder.

As far a mixers go, I always hire (when I'm doing concrete) - and have
found the belle minimix is a great deal more capable than the cheapies.